I realise I have already gone ahead of myself because I haven’t really said why I think the plant I saw yesterday was a)a fern and b)maidenhair spleenwort.
It seems that the definition of a fern is determined by the fact that they do not produce seeds. In other ways they are similar to seed bearing plants, in that they have supporting and conducting (vascular) tissues.
Ferns, because they lack seed, rely on the dispersal of spores. Once dispersed these turn into a prothallus (A separate phase in the life cycle of the fern)in wet conditions. The prothallus then produces male and female sexual organs which reproduce to form the plants we recognise.
So because ferns rely on the dispersal of spores these are often visible, usually underneath the leaves. The spores are contained in a capsule called a sporangium, which are in turn clustered together in sori. How the sori are organised on the leaf is an important diagnostic feature. Often there is a protective cover called an indusium which are also important characteristics.
Plants of the Asplenium genus often have linear sori and indusia , and this is important when first trying to identify the what the leaf might be. (Not all leaves will be fertile but from the top they look the same as the fertile leaves.)
Maidenhair spleenwort has these linear sori, and this combined with the dark rachis and once pinnate leaves make it fairly easy to key out. Unfortunately at this time of the year its not so easy to see the sori or indusium as the sporangia have already expanded to release the spores. Note how the mature sori are clustered towards the apex of the fern leaf.